Coating rate cell

ABSTRACT

An open top coating cell defining a slot. A plurality of apposed ribs are disposed on the inner side walls of the cell to form expansion chambers whereby turbulence is imparted to treating liquid passing through said cell.

o i United States Patent 1 [111 3,731,653 Leland et al. 1 May 8, 1973 COATING RATE CELL [56] References Cited [75] Inventors: James F. Leland, Detroit; William S. UNITED STATES PATENTS Russell, Warren; James G. Mabarak, Grosse Pointe an of Mich Burke Cl 3.1. X 2,456,650 12/1948 Ryan ....118/429 X 3,077,155 2/1963 Maddock et a1... ..95/98 [73] Asslgnee' a, a gimshing cm'pomion 3,362,315 1/1968 Buechner ..95/96 3,373,674 3/1968 Buechner ..95/97 [22] Filed: Apr. 7, 1971 3,427,949 2/1969 Knight et a1. ..95/96 X [21] App1.No.: 132,240

- Primary Exammer-Morr1s Kaplan Related [1.8. Application Data Attorney-William J. Schramm [62] Division of Ser. No. 650,844, July 3, 1967, Pat. No.

3,597,284. 7] ABSTRACT An open top coating cell defining a slot. A plurality of [52] US. Cl. .,...l18/429, 95/89 R apposed ribs are disposed on the inner Side walls of [51] Int. Cl. ..B05c 3/04 the l to fol-m expansion chambers whereby mrbw [58] Field of Search ..1 18/429, 500, 602, lence is imparted to ti liq id passing through 118/603; 95/89 R, 89 L, 89 D, 96, 97, 98, 99, 100; 96/63; 259/4; 134/182 said cell.

1 Claim, 7 Drawing Figures PATENTED HAY 8 I975 sum 1 BF 3 SHEET20F3 II [III] I! II H [I'll H111 PATENTED W- 81973 COATING RATE CELL This is a division of our patent application, Ser. No. 650,844, filed July 3, 1967 and now U.S. Pat. No. 3,597,284.

This invention relates to an improved apparatus for controlling the ratio and concentration of the chemical components of processing solutions for coating metal surfaces.

The chemically reactive processing solutions for coating metal surfaces, such as those solutions which contain hexavalent chromium, contain both coating components and accelerating components. As is known in the art, there is a particular range of concentration of the coating and accelerating components of these solutions which, if maintained, will produce the optimum coating results on the metal surfaces treated. During the coating process, the solution components are depleted both by chemical reaction with the metal surface as well as by mechanical losses, e.g., drag our or entrapment of the solution in and on the metal surface, leaks and the like. Thus, in order to maintain the concentration of these components within the desired range, compensation for all of these depleting factors must be made.

In a copending application Ser. No. 572,907, filed Aug. 17, 1966 and now U.S. Pat. No. 3,481,797, there is disclosed a process whereby a chemically reactive coating solution is operated to compensate for both the mechanical and chemical losses of the components by utilizing two replenishing materials. While both of these replenishing materials contain coating and accelerating components, one is formulated to compensate for mechanical losses in the solution while the second is formulated to compensate for the chemical consumption of the solution by reaction with the metal surface. Thus, by adjusting the ratio of the two solutions which are added to replenish the processing bath, it is possible consistently to maintain the concentration of the bath components within the particular desired range which will produce the optimum coating result.

While the method of this copending application has, in general, been quite satisfactory, it has been found that in some instances, variations in operating conditions of the processing bath have occurred, frequently very gradually over a long period of time, which have affected the rate at which the chemical consumption of the bath components take place. These have included temperature changes in the processing bath, variations in the rate at which the replenishing chemicals are added to the bath, and the like. While some variations of this nature may be tolerated without undue affect on the coating results obtained, where these factors are not discovered and continue for an appreciable period of time, the coating weight produced on the metal surfaces being treated may eventually vary appreciably from that which is desired. Accordingly, it has been necessary frequently to check the weight of coating being produced on the metal surface treated. This, however, has generally involved the destructive testing of a portion of the metal surface and, as such, is not desirable.

It is, therefore, an object of the present invention to provide an improved apparatus for operating chemically reactive processing solutions for treating metal surfaces, whereby the optimum coating results may be maintained without the need for destructive testing of the metal surfaces treated.

A further object of the present invention is to provide an improved testing cell by means of which a determination may be made of the coating weight which is produced on the metal surface being treated without the need for destructive testing of the surface.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

In the drawings which are attached hereto andform a part hereof, FIG. 11 is a schematic representation of a control system for carrying out the process of the present invention; FIG. 2 is a top plan view of a coating weight cell for use in the present process; FIG. 3 is a sectional view of the coating weight cell taken along the lines AA of FIG. 2; FIG. 4 is a sectional view of the coating weight cell taken along the line BB of FIG. 2; FIG. 5 is a top plan view of an alternative configuration of a coating weight cell for use in the present process; FIG. 6 is a sectional view of the coating weight cell taken along the line AA of FIG. 5; and FIG. 7 is a sectional view of the coating weight cell taken along the line BB of FIG. 5.

Pursuant to the above objects, the present invention includes apparatus for forming a chemically reacted coating on a metal surface which comprises contacting the metal surface to be treated with a chemically reactive processing solution containing coating components and activating components, maintaining the processing solution in contact with the metal surface for a period sufficient to form the desired coating thereon, periodically replenishing said processing solution by adding a first replenishing composition containing coating and accelerating components in substantially the same ratio as in the processing solution as originally formulated and a second replenishing composition containing coating and accelerating components in a ratio suitable to compensate for the depletion of these components in the processing solution by reaction with the metal surface being treated, the ratio of the two replenishing solution being such as to reestablish the concentration and ratio of coating and accelerating components in the processing solutions substantially as originally formulated, periodically passing a portion of the processing solution through a test zone, contacting a test specimen having a metal surface with the thuspassed processing solution, within said test zone, determining the weight of coating produced on the metal surface of the test specimen in a given period of contact with the processing solution for the processing solution flow rate used, correlating the weight of coating thusproduced to that being produced on the metal surface treated with the main body of the processing solution and adjusting the ratio of the two replenishing compositions added to the processing solution based on this correlation, so that the concentration and ratio of coating and accelerating components in the processing solution is maintained at a level to produce the desired coating weight on the metal surface being treated. In this manner, variations in the operating conditions of the processing solution which affect the weight of coating produced on the metal surfaces treated are detected, without the need for destructive testing of the production metal surface being treated and compensation for these variations can be made so that the desired coating weight will continually be produced.

More specifically, in the practice of the method utilizing the apparatus of the present invention, it has been determined that the weight of coating formed by a chemically reactive processing solution on a test specimen may be correlated to the coating weight which is actually produced on metal surfaces treated during production with this processing solution. Accordingly, by coating a test specimen, determining the coating weight produced under the particular coating conditions used and then correlating this to the weight of coating produced with the same processing solution in the actual production line, the coating weight produced on the production line can be determined without the necessity of destructive testing of the actual product being produced.

Although the formation of the coating on the test specimen may be effected in the actual production line, it has been found that variations, such as in the flow rate on the production line, build up of impurities such as cleaner residues and the like, may adversely affect the reproducability which is obtained, thus making it difficult to maintain a good correlation between the coating weight on the test specimen and that on the production material. Additionally, it is desirable that a sufficiently high coating weight be formed on the test specimen as to minimize any errors which may occur in weighing, due to the limits of accuracy with which this may be carried out. Often the coating weights produced on the production line are sufficiently low as to make this difficult. Accordingly, in the present method, it is desirable to contact the test specimen with the chemically reactive processing solution in a separate test zone. in this manner, a closer control over the processing solution flow rate, and contact time with the metal surface of the test specimen can be maintained and a heavier, more easily reproducable coating weight is obtained.

In carrying out this method, a metallic surface, such as metallic sheet or strip or formed articles such as cans, are treated with the chemically reactive processing solution by operating in accordance with the procedure which is described in copending application Ser. No. 572,907, filed Aug. 17, 1966. A portion of the processing solution used is passed through a test zone in which there is contained a test specimen having a metallic surface. Desirably, the metallic surface of the test specimen is the same as the metallic surface being treated on the actual production line with the processing solution. Although the coating weight produced on the test specimen can still be correlated with that produced in actual production on a different metal surface, the use of different metal surfaces does introduce additional variables which mustbe taken into account. Accordingly, it is preferred that the metal surface on the test specimen is the same as the metal surface actually being treated with the processing solution on the production line. The test specimen may be of any suitable configuration, although for ease of handling a test specimen in the form of a metal sheet or panel is preferred.

The test zone in which the coating on the test panel is produced may be of any suitable configuration, having means for flowing the processing solution in contact with the metal surface of the panel. A particularly suitable test zone is the coating weight cell, as shown in FIGS. 2 through 7 of the drawings, as will be discussed in more detail hereinafter. The test panel is positioned in the test zone and the processing solution, taken from the main body of the processing solution on the production line is passed in contact with the metal surfaces of the panel for a period of time sufficient to form a chemical coating on the metal surface, at the processing solution flow rate which is used. Desirably, the test panel is maintained in the test zone in contact with the processing solution for a period of from about 30 seconds to 10 minutes, depending upon the makeup of the processing solution which is used. Typically, where the processing solution is one which is suitable for spray or immersion applications of formed articles, a contact time of about 3 minutes has been found to be suitable while contact times of about one minute have been found to be suitable where the processing solution is one used in a strip line.

At the above contact times it has been found that as the flow rate of the processing solution is increased, there is an increase in the weight of coating which is produced on the test panel until substantially constant coating weight conditions are reached. At this point, it is found that further increases in the flow rate of the processing solution do not result in any further appreciable increases in the coating weight in a given time interval. Accordingly, it is generally desirable to use a processing solution flow rate in the test zone which is sufficiently high so as to obtain these conditions. Typically, for the times specified above, flow rates are used which will produce a coating weight which is equivalent to that of at least about 15 milligrams on a 4 inch by 6 inch steel test panel, and preferably about 20 to milligrams. It is, of course, to be appreciated that the actual flow rates used, in terms of actual volume of solution per unit time, will depend upon the configuration of the test zone. it is to be appreciated, of course, that contact times and flow rates which are outside of these typical rangesmay also be used in many instances, to obtain comparable results.

When the desired coating has been formed on the test panel, it is removed from the testing zone and the weight of coating which has been produced is determined. This may be done in any convenient manner, as for example, by weighing the test panel prior to the time it is inserted in the test zone and then reweighing the panel after it is removed from the test zone, the difference in weight being the amount of coating which has been applied to the panel. Alternatively, after removing the panel from the test zone it may be weighed and then the coating formed on the panel surfaces stripped off, following which the panel is reweighed, the difference in weight again being the amount of coating which was produced on the test specimen surface. When using the latter technique, the method of removing the coating from the panel will, of course, depend upon the nature of the coating which has been formed. For example, where the coatings are the type which contain hexavalent chromium, the coating may be stripped from the panel with a concentrated nitric acid solution, for example, a solution having a ratio of nitric acid to water of at least about 1:1. Other methods of stripping the coating from the test panel surface, as are known to those in the art, may also be used. It has been found that the surface of the test panels should be rinsed with water, after being removed from the test zone, regardless of which coating weight determination technique is used.

The correlation of the coating weight produced on the test specimen with the coating weight actually produced on the production line, may be made by, initially, actually determining the coating weight produced on the production line under the particular processing solution and operating conditions which are used. Similar determinations for test panels coated in the test zone are also made, using the same processing solution, and a curve can be drawn plotting the coating weights on the processing line against the coating weights obtained on the test panels. Thereafter, in controlling the operation of the coating process, when the periodic treatment of the test panel in the test zone is effected and the weight of the coating produced on the panel is determined, it may then be correlated to the actual coating weight produced on the production line with this same processing solution by means of the curve which has been plotted. Where this correlation indicates that the actual coating weight produced on the production line has varied from that which is desired, adjustments can then be made in the operating conditions so as to bring the coating weight produced back to the desired level.

It is to be appreciated that the coating weight produced, whether on the test specimen or on the production line, is directly proportional to the time the metal surface is in contact with the coating solution, for a given application method. Thus, for example, with a hexavalent chromium containing coating solution, it is possible to determine the coating weight being produced on the production line without measuring the coating actually produced on the workpieces. This can be determined by knowing the contact time on the line and the coating weight produced on the test specimen in the test zone, for the conditions used, with the same coating solution.

The chemical coating solutions which may be used in the present process are desirably those wherein the components of the replenishing composition may be added as solutions. Although coating solutions wherein the replenishing compositions are added as a dispersion, paste, slurry, powder, or the like, may also be used, for ease and simplicity in handling, the addition of the replenishing compositions in the form of solution is preferred. Exemplary of such coating solutions are the chromate coating solutions, including those based on chromic acid, combinations of chromic acid and phosphoric acid and the like. Typically, these solutions also contain one or more activating materials, as are known in the art, such as fluoride ions. Other coating solutions which may also be used include oxalate coating solutions, e.g., those based on oxalic acid, alkali metal oxide, or the like; phosphate coating solutions, such as those containing alkali metal phosphates, e.g., the so called iron phosphate coating process solutions, zinc acid phosphates, and the like. These coating solutions may be used in the treatment of ferrous metal surfaces, aluminum surfaces and zinc surfaces, to provide a protective and/or paint base coating thereon.

These solutions may be applied to the various metal surfaces to be treated in any convenient manner, as for example, by spraying, immersion, flowing, and the like.

In many instances, the application of the coating solution is preferably carried out either by immersing the metal surfaces to be coated in the solution or by spraying the solutions on the surface.

The control of these processing solutions is effected by the use of two replenishing compositions, one of which is formulated so as to compensate for mechanical losses from the processing solution, e.g., by dragout, or entrapment in an on the metal surface, and the second is formulated to compensate for the chemical consumption of the solution components in reacting with the metal surface being treated. The addition of these two replenishing materials to the processing solution is carried out so that the ratio and concentration of the components in the processing solution will be maintained at substantially the level-.as they are originally formulated. For a detailed description of this control process, as well as the coating solutions which may be used, reference is made to copending application Ser. No. 572,907, filed Aug. 17, 1966, the disclosure of which application is hereby incorporated into the present description.

As has been noted hereinabove, in the practice of the present method, once the determination of the coating weight produced on the metal surfaces of the test specimen has been made and this has been correlated to the coating weight which is actually produced on the production line, and it has been determined that there is a variation from the desired coating weight an adjustment is then made in the replenishing of the processing solution. Desirably, this adjustment is made by changing the amount of the replenishing material which is added to compensate for the chemical consumption of the components of the processing solution. This adjustment results in a change in the ratio of the two replenishing materials which are added, thus establishing a new ratio, and operation with this new ratio is continued until a subsequent change in the coating weight indicates that further adjustment is necessary. In some instances, it may be found that the change in the coating weight produced has been caused by some other factor in the operation of the coating process, as for example a change in flow rate, temperature, or the like. Where this is the situation, the production of the desired coating weight may be reestablished by an adjustment of these operating conditions, where this is practical. Where this can not be done, however, the change in these operating conditions may be compensated for by varying the amount of the chemical replenishing material, thus establishing a new ratio of the mechanical replenishing material to the chemical replenishing material which will produce the desired coating weight on the production line under the same processing conditions.

Referring now the drawings which are attached hereto and form a part hereof, FIG. 1 is a schematic representation of the processing system of the present invention. As shown in this Figure, solution tank 1 contains a chemically reactive coating solution 3. Pump 5 is provided which delivers solution from the tank through lines 7 and 9 to the spray headers 15 which spray the solution on a workpiece 41 to form a chemically reacted coating thereon. A catch basin 17 is provided beneath the spray headers 15 to collect the overspray material and return it to the solution tank 1 through line 19.

A side line 11, having a valve 13, is provided from the main solution line 9, whereby a portion of solution being pumped from the tank I to the headers 15 may be directed through a coating weight cell or test zone 2. Within this test zone, the solution is flowed in contact with a test specimen, to form a coating thereon. The solution overflows at the top of the cell 2, through the serations 12, and is returned to the solution tank 1. Additionally, auxiliary tanks 21 and 23 are provided which contain, respectively, the chemical and mechanical replenishing compositions. Pumps 25 and 27 deliver these replenishing materials, through lines 31 and 35 and 29 and 33, respectively, to the solution tank l. A control means, shown generally as 37 with electric lines 39, is provided whereby the operation of the pumps 25 and 27 are controlled so as to deliver the desired amounts of each of the replenishing materials to the solution tank.

FIGS. 2, 3 and 4 are a top plan view and two sectional views of a coating weight cell for use in the present process. As shown in these figures, the cell 2 is a box-like structure having a bottom member 4, two endwall members 6 and two sidewall members 8. The sidewalls and endwalls are positioned so as to receive a test specimen which is to be contacted with the processing solution, such a test panel. The bottom member is provided with an interior passageway 10 having an opening 14 through one of the end walls, whereby the processing solution is introduced into the test cell. This interior passageway in the bottom member is provided with a series of openings 16 into the main body of the test cell, whereby the processing solution introduced into the passageway 10 is passed into contact with a test specimen positioned within the cell. By providing a multiplicity of these openings along the entire length of the interior passageway, substantially uniform flow of the processing solution in contact with both sides of the test specimen is effected. Means are provided within the cell whereby turbulence is imparted to the processing solution as it is passed in contact with the test specimen, thereby insuring a more rapid reaction of the coating solution with the surfaces of the test panel, by bringing more fresh coating solution in contact with the panel surfaces. As is shown in FIG. 3, this turbulence is imparted to the processing solution by providing an enlarged chamber 18 adjacent the base of the test cell into which the openings in the bottom member, which are at an angle, direct the processing solution, initially, away from the surfaces of the test panel. Desirably, the top edges of the side members of the coating cell are provided with serrations 12, through which the processing solution flows after it has passed through the coating weight cell in contact with the panel surfaces. The number and size of these serrations are selected so as to provide the desired, constant volume and level of coating solution in the cell. By positioning the coating weight cell 2 above the main processing solution tank, as is shown in FIG. 1, the processing solution can be pumped through the coating weight cell, in contact with the test panel surfaces, overflowing through the serrated portions of the side wall members and being returned to the main processing tank. Additionally, the upper portion of the side wall members 8 and end wall members 6 are desirably provided with a chamber 20, to make insertion of the test panel in the cell easier.

Referring now to FIGS. 5, 6 and 7 of the drawing, these show a top plan view and two sectional elevations, of a modified form of the coating weight cell 2. In this modification, which is generally similar to that shown in FIGS. 2 through 4, an interior passageway 10 is formed in the bottom of the cell by means of a perforated member 22 which is positioned above the bottom member 4 of the cell thus forming the passageway 10 into which the processing solution is introduced through the opening 14 in one end wall member 6. Additionally, the side wall members 8 are provided with a series of projecting rib members 24 which are substantially parallel to the bottom member of the cell and are oppositely disposed on each of the side wall members 8. These rib members 24, with the side wall members 8, form a series of chambers throughout the heigth of the cell. In this manner, the desired turbulence is imparted to the processing solution as it flows from the interior passageway, through the openings 16 in the perforated member 22, upwardly between the two sidewall mem bers of the cell, in contact with the test panel. Here again, by providing two rows of perforations 16 through which the processing solution passes, a substantially uniform flow of the processing solution onto both sides of the test panel is achieved. Additionally, the upper edges of the side wall members are provided with serrations 112 through which the coating solution overflows out of the cell. Desirably, the upper portion of the side wall members in this variation are also provided with an offset section26 to form an enlarged zone or chamber which, with the chamber 20 on the uppermost rib members on both side wall members make insertion of the test panel into the cell easier.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, parts and percents are by weight and temperatures are in degrees centigrade. It is to be appreciated, however, that these examples are merely exemplary of the present invention and the manner in which it may be practiced and are not to be taken as a limitation thereof.

EXAMPLE I A treating solution was formulated containing the following components in the amounts indicated:

Components of Weight H O balance The pH of this solution was 1.7 and it had a concentration of 15 points.

Replenishing solutions were, also formulated, as follows, for replenishing the physical and chemical losses of the treating solution:

SOLUTION 1 (Physical) Components by Weight H PO (75% aqueous solution) 63.4

HF aqueous solution) 3.72

H,O balance SOLUTION 2 (Chemical) Components by Weight CrO 10.3 H PO (75% aqueous solution) 11.2 HP (70% aqueous solution) 17.0 H O balance Using a conventional commercial spray strip line, 5052 aluminum alloy strip was coated using the following stages:

1. Alkaline cleaner 15 second spray at 71C 2. Alkaline cleaner second spray at 71C 3. Cold water spray rinse 4. Treating solution (as set forth above) 8 second spray at 54C 5. Water spray rinse 6. Deionized water spray rinse The alumnum alloy strip was passed through at a rate of about 250 feet per minute. The conductivity of the treating solution applied in stage four was measured to determine the concentration of the solution. Based on this measurement and the line speed used, the desired concentration and ratio of the components in the treating solution were maintained by adding the two replenishing materials in the ratio of 1 part of the first to 9 parts of the second.

A by-pass from the pump supplying treating solution to the fourth stage sent a flow of the solution through a coating rate cell, having a structure as shown in FIGS. 5, 6, and 7. The flow through the cell was adjusted to about 2,000 milliliters per minute. A 4 inch by 6 inch panel of the same 5052 aluminum alloy being treated in the line was placed in the cell in contact with the solution for 1 minute. The panel was then removed, water rinse, air dried and weighed. After stripping the panel in concentrated nitric acid (1:1 HNO it was reweighed and the coating weight was found to be 30 milligrams. This was equivalent to a coating weight of 90 milligrams per square foot. From previous observations, it has been determined that, for the same contact time, the coating weight produced on the strip line by spraying was 1.5 times that produced on the panel in the coating rate cell by immersion. Thus, in 1 minute on the strip line a coating of 135 milligrams per square foot would be produced, so that in the 8 second spray time actually used, a coating weight of 18 milligrams per square foot was produced on the strip line. Accordingly, each milligram per square foot of coating produced on the actual production line is equivalent to 1.66 milligrams of coating produced on the test panel in the coating weight cell. The above was repeated periodically as a check on the coating weight being produced on the line and as deviations from the desired coating weight were noted, the ratio of the two replenishing materials was changed to compensate therefore, thus re-establishing the proper concentration and ratio of treating solution components to obtain the desired coating weight on the line of 18 milligrams per square foot.

EXAMPLE 2/ Components CrO H,PO

% by Weight 0.15 0.90

F 0.08 Al 0.02 Cr 0.02 H,O balance The pH of this solution was 2.1 and it has a concentration of 5 points.

Additionally, the replenishing solutions used were as follows:

SOLUTION 1 (Physical) Components by Weight H O balance SOLUTION 2 (Chemical) Components by Weight CrO; 4.96

H PO 1.66

H 0 balance The treating solution was applied by spraying on aluminum cans in a conventional spray can line using the following stages:

1. Acid cleaner 20 second spray at 65C 2. Cold water spray rinse 3. Treating solution (as set forth above) 9 second spray at 54C 4. Water spray rinse 5. Deionized water spray rinse The cans, on a conveyor, were passed through the line at the rate of about 250 cans per minute. The conductivity of the treating solution was measured as in the previous example and based on this and the number of cans treated, the solution was maintained at the desired concentration by adding the two replenishing compositions in the ratio of 9 parts of the first to 1 part of the second.

The coating rate was determined, using the coating rate cell, in the same manner as in Example 1, with the exception that the panel was maintained in contact with the treating solution in the cell for 3 minutes. The coating weight produced on the 4 inch by 6 inch panel was 30 milligrams, which was determined to be equal to a coating weight on the cansof 6.7 milligrams per square foot. Thus, each 1 milligram per square foot of coating on the cans was equivalent to 4.5 milligrams of coating on the test panel. The testing procedure was repeated periodically and the ratio of the two replenishing materials varied as indicated by the test results to maintain desired concentration and ratio of components in the treating solution, as in Example 1.

While there have been described various embodyments of the invention, the compositions and methods described are not intended to limit the scope of the invention, as it is realized that changes therewithin are possible and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A coating weight cell useful in the operation of chemically reactive processing solutions for treating metal surfaces comprising:

means forming the castellations; and

said side walls including a plurality of pairs of opposed rib means, each rib disposed within the cell and extending transversely thereto, whereby to form a series of expansion chambers so as to impart turbulence to the treating fluid passing therethrough. 

1. A coating weight cell useful in the operation of chemically reactive processing solutions for treating metal surfaces comprising: an open top box-like structure having a bottom member, end walls and castellated side walls forming a slot wherein to closely receive a test specimen, said bottom comprising a tubular member having connecting means to receive a treating liquid therein and a perforated top wall portion whereby said liquid is passed into said cell and into contact with said surfaces and discharged through slot means forming the castellations; and said side walls including a plurality of pairs of opposed rib means, each rib disposed within the cell and extending transversely thereto, whereby to form a series of expansion chambers so as to impart turbulence to the treating fluid passing therethrough. 